Lump or agglomerated or shaped materials in any desired shape, such as limestone, ore pellets, fire-clay, and waste materials, are often thermally treated on travelling grates in that hot gases are passed through the charge on the travelling grate so that the charge is heated to predetermined temperature. The charge bed is substantially cooled by cold gases passed through the bed. The cooling gases which have been heated are supplied to the heat treating zone and used as a primary gas and/or secondary gas for the burners. The heat treating zone usually comprises a drying zone, a heating-up zone and a burning zone, each of which may be subdivided. The cooling zone is also subdivided in most cases.
It is known from U.S. Pat. No. 3,172,754 in connection with the hard-burning of pellets to provide over the first cooling zone a gas hood, which opens into a continuous duct, which extends over the afterburning zone, burning zone and heating-up zone. That duct communicates with said zones through openings provided in the roof structure over said zones. The hot cooling gases flow through said openings as secondary air and mix with the flue gases which flow from the laterally disposed burners into said zones. Whereas the continuous duct eliminates the need for separate gas lines, the mixing of the flue gases and the secondary air is poor so that the temperature distribution and the distribution of oxygen in the gases and in the bed are also adversely affected.
It is known from U.S. Pat. No. 3,620,519 to provide a continuous gas hood over the first cooling zone, the burning zone and the heating-up zone. In said gas hood, another, lower gas hood extends over the afterburning zone and a preliminary cooling zone and receives cooling air which has flown upwardly through the charge bed and delivers said cooling air to pass downwardly through the charge bed in the afterburning zone. The heated cooling air which rises into the continuous gas hood in the first cooling zone is heated up by laterally disposed burners and is used as secondary air for burners. The burners are disposed over the first cooling zone or in the passage over the second, lower gas hood. The heated gases then flow under the gas hood into the burning and heating-up zones and are passed downwardly through the bed in said zones. A partial transverse wall is provided between said zones. In said process, all gases are heated up in the cooling zone to the highest temperature required and must subsequently be cooled, e.g., in the heating-up zone to the temperature which is required there, by an addition of cold air. As a result, gas at the maximum volumetric rate becomes available in the cooling zone, the continuous gas hood must have a correspondingly large cross-section. The heat losses at the walls are large, and a cover consisting of a second gas hood is required over the afterburning zone and causes a deposition of dust and a formation of crusts.
European Pat. No. 0 030 396 discloses that at least part of the fuel supplied from the outside to the burning process is charged as solid fuel onto the surface of the pellet bed and that a continuous gas hood is provided over the first cooling zone, the burning zone and the heating-up zone. If heating is effected only by a combustion of the solid fuel supplied onto the bed, the gas hood will have no internal fixtures. If only part of the heat required is supplied by the combustion of that solid fuel, then internal fixtures will be provided over the heating-up and burning zones and said internal fixtures will be provided on their side walls with combustion chambers, which communicate with the gas hood through ducts, through which heated cooling air supplied as secondary air to the combustion chambers. Advantages afforded by both embodiments reside in that the combustion of the solid fuel results in a highly uniform heating of the gases and that the volumetric gas rate is relatively low. But any burners which are used can be operated only under a lower load and nevertheless there will be a certain problem regarding the deposition of ash in the ducts. In both cases, inexpensive solid fuels may be used and the energy costs will be lowered in dependence on the rate at which solid fuel is charged. On the other hand, solid fuel is not available in all cases and it may be desired to permit a selective use of liquid or gaseous fuel.